My MC owns an used bookstore in our world. She's also starting to get additional inventory from supernatural means. She wants to sell these items. Do you have to be able to produce receipts for inventory purchases? Or just that cash went out to make the purchase?
My gut tells me that if she gets audited, she needs to be able to produce receipts. Otherwise she'd be thought to be selling stolen goods (which they aren't, she paid for them, just not in our current world).
So, do you need receipts? If so, and she doesn't have ones that the IRS would accept, would it be plausible to falsify receipts? Is there a threshold at which they'd give a pass (if she has to stay under $50k in income from the sales of these items, she can work with that)?
I've run a "small used bookstore" online, though not had enough income to get anyone's attention. I've also been on discussion boards with many "real" used booksellers.
It really depends on the prices and valies of the books, especially what the bookseller generally pays when buying books. Regardless, the amount spent on buying inventory can be deducted from reported profits, but one generally needs receipts for buying the books.
For most "average" used bookstores, large volumes of books are bought at library book sales and such, at prices from 25 cents to two or three dollars each. Often customers will bring in books for sale/trade, even boxes at a time. Books brought in are usually for store credit only, but some bookstores pay cash. People have been known to GIVE boxes of books to booksellers (ahem, and they tend not to be very sellable either). These are the kinds of books (that generally sell for $1 to $20, though an occasional rare and valuable book is found this way), and it's really not that big a deal.
If the bookstore deals more in valuable and collectible books (something it sounds like your bookseller might be doing), things get complicated. Is it possible to get a sales recept for these "supernatural" books from a "legit" source? If not, the bookdealer can just take the loss of the books' purchase prices, and there shouldn't be a problem unless she's audited. An auditor might ask "I see you've got these fancy books for sale - where did you get them, and what did you pay for them?" She wouldn't have any good answers.
But there would be no evidence of her stealing books (though perhaps a strong suspicion - there are books so rare that there are a dozen or less known copies worldwide, and if she "comes up" with an extra copy, those places will be checked to see if THEIR physical copies are still there, and even if they are, she will be strongly questioned about where she got the book) unless one of her books actually matched the description of a book that had been reported stolen. Whenever a valuable book is stolen from a bookseller or library, notice goes out. I'm sure valuable stolen books were listed in AB Bookman's, the used-book industry newsletter/magazine for decades (no lnger published), but now they would be listed online and easily findable by searching for title/author.
If these books (the titles and/or authors) don't exist in "our world" except in her bookshop, sooner or later (and probably sooner) someone's going to look one up online, not find it, put pics and scans online and ask lists of hundreds of online booksellers (some of whom have been dealing in rare books for many decades), and not finding another copy, will have a real mystery on their hands. A few books are hand-made and only one copy made, but that would have obvous differences (to an expert) from a commercially made book.
Obligatory bookseller novel references: John Dunning's "Cliff Janeway" mystery series, "Booked To Die," "The Bookman's Wake", etc.
Was that more than you wanted to know?